Fuel cell systems in many cases use fuel cells in which an electrolyte membrane is sandwiched between a fuel electrode and oxidant electrode; and water is generated at the oxidant electrode at the same time as electric power is generated by an electrochemical reaction between hydrogen supplied to the fuel electrode and oxygen in the air supplied to the oxidant electrode.
In such fuel cells, a predetermined voltage and electric current cannot be output under operation in a temperature lower than a normal operation temperature. Furthermore, when freezing occurs with a temperature falling below the freezing point during a non-operation period, in many cases, a warm-up operation is performed after a start-up until a normal operation temperature is reached. As a method of the warm-up operation, a low-efficiency operation is performed in which an amount of air supplied to a fuel cell is reduced to warm-up the fuel cell by using the increased heat loss.
In such a warm-up operation, when the amount of hydrogen supplied to a fuel cell becomes insufficient, there is a risk that a catalyst may be lost by carbon oxidation or a membrane may be damaged due to generated heat. When such a shortage of hydrogen occurs in a fuel cell, the voltage of the fuel cell becomes negative. Therefore, it has been suggested that the warm-up operation be inhibited or output of the fuel cell be limited when the voltage becomes negative due to a shortage of hydrogen gas during a warm-up operation of fuel cell (for example, refer to Patent Document 1).
It has been also suggested that when a fuel cell comprising a plurality of unit cells is started-up under a low temperature, a fuel cell degradation due to a shortage of gas is suppressed and the warm-up of fuel cell is performed in a short period of time even when a gas flow channel is clogged due to freezing by providing, with each of the unit cells, a cell stoichiometric ratio calculator for calculating a stoichiometric ratio of a certain gas and a gas amount increasing unit for increasing an amount of supply of the certain gas when the obtained stoichiometric ratio is lower than a predetermined value (for example, refer to Patent Document 2).
Further, a method has been suggested for blowing condensate in a gas flow channel by partially recycling anode-off gas through an anode off-gas recycle line when the amount of fuel gas flow supplied to a fuel cell is small and a gas flow channel at an anode side is clogged due to the condensate (for example, refer to Patent Document 3).
Furthermore, by using that a power generation efficiency is reduced when a supply pressure of hydrogen gas to a fuel cell is lowered, another method has suggested for performing an warm-up operation of a fuel cell by lowering a supply pressure of hydrogen gas to a fuel cell at a startup of the fuel cell than in a normal operation (for example, refer to Patent Document 4).